Photovoltaic modules in field-use suffer from performance loss over time due to dirt build up on the primary surface. The problem is even more severe in areas receiving little rainfall—often the same areas where cloudless days help maximize the available solar energy.
While glass is often used as the transparent glazing on many rigid PV units, it suffers from both a weight issue and inflexibility issue, and therefore is not useful in flexible PV applications.
Polyvinylidene fluoride (PVDF) is known to be highly chemical resistant, relatively inert, and having a very low surface energy that little can stick to. The low surface energy means that PVDF materials readily can shed dirt and grime. The use of a PVDF film over a rigid thermoplastic support layer in a photovoltaic module is described in PCT patent application PCT/US07/74538.
WO2007085769 describes multi-layer PVDF films used for laminates of multi-layer film constructions where it is laminated to PET or PEN.
A current trend in photovoltaic module is for “flexible” photovoltaic modules, which can be transported in a roll, and where all the components in the module (back substrate, solar cell, encapsulant and front glazing) need to be flexible. Expensive fluoropolymers such as ethylyene tetrafluoro ethylene (ETFE) are being employed for this application. These polymers are have excellent low-temperature impact resistance. However—there is a need for lower cost alternatives that can meet the performance requirements of this demanding application.
The glazing for a flexible photovoltaic module must have extended durability under solar radiation (especially UV), high transmittance of solar energy, the ability to adhere to the encapsulants which today are primarily peroxide cross-linked EVA, good dirt shedding and dirt release performance, the ability to not support mold or bacterial growth, the ability to withstand impacts from hail, the ability to withstand high temperature curing temperatures used to form the module without deformation and discoloration. Further, the glazing is ideally very thin—to lower the weight and the cost of the PV module.
This invention claims a constructions of PV modules that use specific PVDF films, coatings or laminates, for the front glazing of photovoltaic modules. Modules can be flexible, or rigid. The proper PVDF films, coatings or laminates can surprisingly meet all the requirements.
While PVDF is known to have very good weathering, it is also generally known to have poor cold temperature impact performance, which presents a problem during a hail storm and other low temperature impact.
It has now been found that properly formulated PVDF film, coatings or laminates can provide all of the key performance properties needed for a flexible glazing, as noted above, as well as offering the potential to produce lower cost constructions. Laminates adhered to an ethylene vinyl acetate (EVA), are especially effective. It has been found that the final laminate can withstand cold temperature impact due to the flexibility of the Kynar layers combined with the toughness of the EVA and/or laminating film. Ultimately, this now offers a lower cost option to todays high cost monolayer films.